U.S. patent application number 10/703072 was filed with the patent office on 2004-06-17 for electrode for solid polymer fuel cell.
Invention is credited to Fukuda, Kaoru, Inai, Shigeru, Iwasawa, Chikara, Kaji, Hayato, Kohyama, Katsuhiko, Muro, Takeshi, Shinkai, Hiroshi, Tani, Masaki, Watanabe, Shinya.
Application Number | 20040115517 10/703072 |
Document ID | / |
Family ID | 32510582 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040115517 |
Kind Code |
A1 |
Fukuda, Kaoru ; et
al. |
June 17, 2004 |
Electrode for solid polymer fuel cell
Abstract
An electrode for a solid polymer fuel cell includes a gas
diffusion layer, an electrode catalyst layer disposed between a
solid polymer membrane of the fuel cell and the gas diffusion
layer, and a water-holding layer disposed between the gas diffusion
layer and the electrode catalyst layer. Under high-relative
humidity conditions of reaction gases, flooding can be prevented
because the electrode catalyst layer is made porous, while under
low-relative humidity conditions of reaction gases, sufficient
water contents can be stably provided thanks to the water-holding
layer so that proton conductivity of the solid polymer membrane can
be maintained appropriately. Consequently, high-performance and
high-durability electrode and membrane electrode assembly for a
solid polymer fuel cell can be provided such that the performance
and the durability thereof are not affected by change in relative
humidity in reactant gases supplied to the solid polymer fuel
cell.
Inventors: |
Fukuda, Kaoru; (Saitama,
JP) ; Tani, Masaki; (Saitama, JP) ; Inai,
Shigeru; (Saitama, JP) ; Kaji, Hayato;
(Saitama, JP) ; Iwasawa, Chikara; (Saitama,
JP) ; Watanabe, Shinya; (Saitama, JP) ;
Kohyama, Katsuhiko; (Saitama, JP) ; Shinkai,
Hiroshi; (Saitama, JP) ; Muro, Takeshi;
(Saitama, JP) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Family ID: |
32510582 |
Appl. No.: |
10/703072 |
Filed: |
November 7, 2003 |
Current U.S.
Class: |
294/17 ;
429/481 |
Current CPC
Class: |
H01M 8/1007 20160201;
H01M 4/921 20130101; H01M 4/8605 20130101; Y02E 60/50 20130101;
H01M 4/8821 20130101; H01M 8/04097 20130101; H01M 8/04291 20130101;
H01M 8/0234 20130101; H01M 4/926 20130101; H01M 8/04104 20130101;
H01M 4/8636 20130101 |
Class at
Publication: |
429/044 ;
429/042; 429/030 |
International
Class: |
H01M 004/94; H01M
004/96; H01M 008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2002 |
JP |
2002-325400 |
Nov 8, 2002 |
JP |
2002-325401 |
Claims
What is claimed is:
1. An electrode for a solid polymer fuel cell comprising: a gas
diffusion layer for uniformly diffusing reactant gases; an
electrode catalyst layer disposed between a solid polymer membrane
of the fuel cell and the gas diffusion layer to support a catalyst
for reaction of the diffused reactant gases; and a water-holding
layer disposed between the gas diffusion layer and the electrode
catalyst layer to enhance a water-retaining capability of the gas
diffusion layer, wherein the electrode catalyst layer is made
porous to facilitate drainage of water from the electrode catalyst
layer.
2. An electrode according to claim 1, wherein the gas diffusion
layer has a moisture content ranging between 50% and 90% inclusive,
the moisture content being obtained by an equation: 4
MoistureContent = SM - DM DM .times. 100 [ % ] where SM denotes
mass of the gas diffusion layer under saturation of water vapor
pressure; and DM denotes mass of the gas diffusion layer under dry
conditions.
3. An electrode according to claim 1, wherein a differential
pressure of the reactant gases calculated from two pressures
measured at places upstream and downstream of the gas diffusion
layer when a predetermined flow rate of the reactant gases pass
through the gas diffusion layer ranges between 60 mmH.sub.2O and
120 mmH.sub.2O inclusive.
4. An electrode according to claim 1, fiber including a
water-repellent layer provided between the water-holding layer and
the gas diffusion layer to facilitate drainage of water from the
gas diffusion layer.
5. An electrode according to claim 1, wherein the electrode
catalyst layer includes a catalyst, a carbon powder for supporting
the catalyst, an ion conductive polymer, and a crystalline carbon
fiber; and wherein the gas diffusion layer includes, in sequence
from an electrode catalyst layer side: a water-repellent layer
containing an ion conductive polymer, a carbon powder and a
crystalline carbon fiber; and a carbon cloth layer.
6. A membrane electrode assembly for a solid polymer fuel cell
comprising: a pair of electrodes; and a solid polymer membrane
disposed between the electrodes, wherein at least one of the
electrodes includes: a gas diffusion layer for uniformly diffusing
reactant gases; an electrode catalyst layer disposed between the
solid polymer membrane and the gas diffusion layer to support a
catalyst for reaction of the diffused reactant gases; and a
water-holding layer disposed between the gas diffusion layer and
the electrode catalyst layer to enhance a water-retaining
capability of the gas diffusion layer, wherein the electrode
catalyst layer is made porous to facilitate drainage of water from
the electrode catalyst layer.
7. A membrane electrode assembly according to claim 6, wherein the
gas diffusion layer has a moisture content ranging between 50% and
90% inclusive, the moisture content being obtained by an equation:
5 MoistureContent = SM - DM DM .times. 100 [ % ] where SM denotes
mass of the gas diffusion layer under saturation of water vapor
pressure; and DM denotes mass of the gas diffusion layer under dry
conditions.
8. A membrane electrode assembly according to claim 6, wherein a
differential pressure of the reactant gases calculated from two
pressures measured at places upstream and downstream of the gas
diffusion layer when a predetermined flow rate of the reactant
gases pass through the gas diffusion layer ranges between 60
mmH.sub.2O and 120 mmH.sub.2O inclusive.
9. A membrane electrode assembly according to claim 6, further
including a water-repellent layer provided between the
water-holding layer and the gas diffusion layer to facilitate
drainage of water from the gas diffusion layer.
10. A membrane electrode assembly according to claim 6, wherein the
electrode catalyst layer includes a catalyst, a carbon powder for
supporting the catalyst, an ion conductive polymer, and a
crystalline carbon fiber; and wherein the gas diffusion layer
includes, in sequence from an electrode catalyst layer side: a
water-repellent layer containing an ion conductive polymer, a
carbon powder and a crystalline carbon fiber; and a carbon cloth
layer.
11. A membrane electrode assembly according to claim 6, a
percentage of a charge amount of catalytic substances existing on
an interface between the solid polymer membrane and the electrode
catalyst layer to a charge amount of all catalytic substances
existing in the electrode catalyst layer is 15% or greater, the
charge amounts being determined by a cyclic voltammetry.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to electrodes or membrane electrode
assemblies for a solid polymer fuel cell (SPFC; also called
"polymer electrolyte fuel cell" or PEFC), and more particularly to
an electrode capable of achieving a stabilized power-generation
performance even under conditions of unstable humidity in reactant
gases supplied to solid polymer fuel cells, and preventing
deterioration derived from delayed supply of the reactant
gases.
[0002] The solid polymer fuel cell has been attracting widespread
attention in recent years as being a power source for electric
vehicles, and the like. The solid polymer fuel cell can generate
electric power at ordinary (sufficiently low) temperatures, and is
thus finding various practical applications.
[0003] The fuel cell includes an anode and a cathode. The anode is
a fuel-gas terminal to which a fuel gas containing hydrogen is
supplied. The cathode is an oxidant-gas terminal to which oxidant
gas containing oxygen is supplied A chemical reaction then takes
place between oxygen in the cathode and hydrogen in the anode,
thereby generating electricity. For example, when air is supplied
as the oxidant gas to the cathode, chemical energy is converted
into electric energy to be supplied to an external load as
expressed by the following equations:
At the anode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.-
At the cathode: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
Overall; 2H.sub.2+O.sub.2.fwdarw.2H.sub.2O (1)
[0004] There is a solid polymer membrane (electrolyte membrane)
between the anode and the cathode of the fuel cell. Protons
generated during reaction in the anode pass through the solid
polymer membrane, and travel with moister to the cathode. Electrons
generated during the same reaction in the anode are carried through
an external circuit to the cathode. The protons and electrons as
thus put together in the cathode react with oxygen in the air to
make water.
[0005] In the solid polymer membrane fuel cell (also called "proton
exchange membrane fuel cell" or PEMFC), moisture should be supplied
to constantly maintain proton conductivity of the solid polymer
membrane (electrolyte membrane), and thus the reactant gases to be
supplied to the fuel cell are humidified in advance.
[0006] In general, the solid polymer fuel cell has a layered
structure as shown in FIG. 6 in which a single cell 100 is
schematically illustrated. Onto both sides of a solid polymer
membrane 101 are provided electrode catalyst layers 102a, 102b, and
on the outsides thereof are provided gas diffusion layers 103a,
103b, to form a membrane electrode assembly (MEA). On both sides of
the MEA are then provided separator plates 104a, 104b, which not
only serve to separate each cell but also serve as is manifolds to
distribute reactant gases such as fuel gases and oxidant gases
between and within the cells. The single cell 100 is formed by
sandwiching the above layers between the separator plates 104a,
104b and holdings the layered structure from outside the separator
plates 104a, 104b. The electrode catalyst layer 102a or 102b and
the gas diffusion layer 103a or 103b make up an electrode (anode or
cathode).
[0007] It is assumed that a shortage of fuel gases encountered
during the process as represented by Equation (1) above would cause
corrosion of carbon in the gas diffusion layers 103a, 103b, as
expressed by Equation (2) as follows:
2H.sub.2O+C.fwdarw.4H.sup.++CO.sub.2.Arrow-up bold. (2)
[0008] If corrosion proceeded as above, catalyst supporting carbon
black would be consumed; this would disadvantageously deteriorate
the membrane electrode assembly, and eventually deteriorate the
fuel cell itself.
[0009] Several attempts have been made to prevent such corrosion of
carbon from proceeding and to eliminate the resulting
disadvantages, mostly with consideration given to the reaction as
in Equation (1); for example, Applicant previously has devised an
approach of giving water-retaining capability to the electrode
catalyst layers (see JP 2003-168442 A). Another approach disclosed
in WO 01/15254 A is to add a catalyst for accelerating electrolysis
of water to the electrode catalyst layers.
[0010] The above existing approaches of giving water-retaining
capability or adding a catalyst for accelerating electrolysis of
water to the electrode catalyst layers would be effective for a
transient shortage of fuel gases, but repeatedly encountered
shortages of fuel gases (e.g., due to abrupt acceleration, or the
like) which would be assumed in actual driving situations, or rated
driving conditions, would disadvantageously result in flooding due
to the enhanced water-retaining capability in the electrode
catalyst layers. The flooding is a phenomenon in which water is
retained in gas diffusion channels such as pores formed in the
electrode catalyst layers and inhibits diffusion of gases. Flooding
would not only lower the performance of the membrane electrode
assembly but also inhibit supply of fuel gases under operating
conditions of the fuel cell such that shortages of fuel gases are
repeatedly encountered, and would expand a region in which fuel
gases are insufficient in the anode, so that corrosion of carbon
could proceed, thus decreasing the performance of the membrane
electrode assembly.
[0011] In order to avoid causing power generation performance of
the fuel cell to lower due to flooding within the cell, a
pore-making material may be added to form a porous structure of the
electrode catalyst layers 102a, 102b which serves to remove water
in the cell (see JP S180879 A). As the gas diffusion layers 103a,
103b, a porous material having a current-collecting property may be
provided on the outsides of the electrode catalyst layers 102a,
102b; for example, carbon paper having a porosity of 80%, etc. may
be employed.
[0012] The pore-making material added to the electrode catalyst
layers 102a, 102b would indeed improve the power generation
performance under high-humidity conditions where a plenty of water
exists in the cell 100 because the pore-making material in the
electrode catalyst layers 102a, 102b would facilitate drainage of
water from the electrode catalyst layers 102a, 102b, thus serving
to prevent flooding; however, under low-humidity conditions, only
adding the pore-making material to the electrode catalyst layers
102a, 102b would rather lead to disadvantageous effects of lowering
the power generation performance because water required to maintain
proton conductivity of the solid polymer membrane 101 would be
drained out through pores formed by adding the pore-making
material.
[0013] The present invention has been made to address the
above-described disadvantages.
SUMMARY OF THE INVENTION
[0014] In one aspect of the present invention, there is provided an
electrode (and, a membrane electrode assembly) for a solid polymer
fuel cell. The electrode comprises: a gas diffusion layer for
uniformly diffusing reactant gases; an electrode catalyst layer
disposed between a solid polymer membrane of the fuel cell and the
gas diffusion layer to support a catalyst for reaction of the
diffused reactant gases; and a water-holding layer disposed between
the gas diffusion layer and the electrode catalyst layer to enhance
a water-retaining capability of the gas diffusion layer. In
addition, the electrode catalyst layer is made porous to facilitate
drainage of water from the electrode catalyst layer.
[0015] With the above arrangement, (1) under high-relative humidity
conditions of reaction gases, `flooding` can be prevented because
the electrode catalyst layer is mad porous (e.g., by addition of
pre-making materials). Therefore, the fuel cell incorporating an
electrode having a structure as above according to the present
invention can maintain a high level of power generation
performance; and (2) under low-relative humidity conditions of
reaction gases, sufficient water contents can be stably provided so
that proton conductivity of the solid polymer membrane can be
maintained appropriately, because the water-holding layer for
enhancing the water-retaining capability of the gas diffusion
layer. Therefore, the fuel cell incorporating an electrode having a
structure as above according to the present invention can achieve
improvement in power generation performance. Moreover, corrosion of
carbon, which would otherwise proceed when a shortage of fuel gases
is encountered, can be inhibited. Consequently, a high-performance
and high-durability electrode and membrane electrode assembly for a
solid polymer fuel cell can be provided such that the performance
and the durability thereof are not affected by change in relative
humidity in reactant gases supplied to the solid polymer fuel
cell.
[0016] Preferably, the above gas diffusion layer may be configured
to have a moisture content ranging between 50% and 90% inclusive.
The moisture content is obtained by the following equation: 1
MoistureContent = SM - DM DM .times. 100 [ % ]
[0017] where SM denotes mass of the gas diffusion layer under
saturation of water vapor pressure; and DM denotes mass of the gas
diffusion layer under dry conditions. By limiting the moisture
content of the gas diffusion layer within the preferable range as
above, deterioration of the electrode (or membrane electrode
assembly) due to shortage of fuel gases can be prevented. To be
more specific, flooding caused by an excessive moisture content of
the gas diffusion layer can be prevented, and corrosion of carbon
in the electrode (or membrane electrode assembly) caused by an
insufficient moisture content of the gas diffusion layer can also
be prevented.
[0018] Moreover, the gas diffusion layer may be configured to have
a desirable range of a differential pressure of the reactant gases.
The differential pressure can be calculated from two pressures
measured at places upstream and downstream of the gas diffusion
layer when a predetermined flow rate of the reactant gases pass
through the gas diffusion layer. The desirable differential
pressure ranges between 60 mmH.sub.2O and 120 mmH.sub.2O inclusive.
Assuming that the moisture content of the gas diffusion layer were
kept constant, change in differential pressure of reactant gases
across the upstream and downstream of the gas diffusion layer would
greatly affect the power generation performance associated with the
change in relative humidity of the reactant gases. In this respect,
by limiting the differential pressure to a desirable range as
above, stable power generation performance of the fuel cell can be
ensured regardless of change in relative humidity of the reactant
gases, and deterioration of the electrode is (or membrane electrode
assembly) due to shortage of fuel gases can be prevented. To be
more specific, flooding caused by an excessive differential
pressure of the gas diffusion layer can be prevented, and corrosion
of carbon in the electrode (or membrane electrode assembly) caused
by an insufficient differential pressure of the gas diffusion layer
can be prevented.
[0019] The electrode may further include a water-repellent layer
provided between the water-holding layer and the gas diffusion
layer to facilitate drainage of water from the gas diffusion layer.
More specifically, the electrode catalyst layer may include a
catalyst, a carbon powder for supporting the catalyst, an ion
conductive polymer and a crystalline carbon fiber. Further, the gas
diffusion layer may include, in sequence from an electrode catalyst
layer side: a water-repellent layer containing an ion conductive
polymer, a carbon powder and a crystalline carbon fiber; and a
carbon cloth layer. Thus-provided water-repellent layer serves to
facilitate drainage of condensed water in the porous gas diffusion
layer in which humidified reactant gases supplied from a separator
plate of the fuel cell are diffused and transferred to the
water-holding layer disposed between the gas diffusion layer and
the electrode catalyst layer, such that water can be supplied for a
relatively short time from the gas diffusion layer to the
water-holding layer. Accordingly, proton conductivity of the solid
polymer membrane can be maintained with sufficient water supplied
through the water-holding layer. Consequently, the power generation
performance can be improved, and corrosion of carbon due to a
shortage of fuel gases can be prevented from proceeding in the
cell.
[0020] In the membrane electrode assembly for the solid polymer
fuel cell according to the present invention, a percentage of a
charge amount of catalytic substances existing on an interface
between the solid polymer membrane and the electrode catalyst layer
to a charge amount of all catalytic substances existing in the
electrode catalyst layer may preferably be 15% or greater.
Hereupon, the charge amounts may be determined by a cyclic
voltammetry. The above percentage of the charge amount determined
by the cyclic voltammetry is an indicator of an adhesion rate
between the solid polymer membrane and the electrode catalyst
layer. If the percentage is 15% or greater (i.e. the adhesion rate
is sufficiently high), decrease in the amount of water reversely
diffused from the cathode as a result of insufficient adhesion rate
can be prevented, and thus corrosion of carbon which would
otherwise proceed can be prevented in the fuel cell.
[0021] Other objects and further features of the present invention
will become readily apparent from the following description of
preferred embodiments with reference to accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic diagram of a single cell of a solid
polymer fuel cell including an electrode according to one
exemplified embodiment of the present invention.
[0023] FIG. 2 is a graph showing a relationship between a moisture
content of a gas diffusion layer of an electrode according to a
second embodiment of the present invention and an undervoltage
(difference between terminal voltages; "A terminal voltage") of a
single cell exhibited before and after an endurance test.
[0024] FIG. 3 is a graph showing a relationship between a moisture
content of a gas diffusion layer of an electrode according to a
third embodiment of the present invention and a terminal voltage of
a single cell.
[0025] FIG. 4A is a graph showing a relationship between a
differential pressure of reactant gases calculated from two
pressures at places upstream and downstream of a gas diffusion
layer of an electrode according to a fourth embodiment of the
present invention and an undervoltage (difference between terminal
voltages; ".DELTA.terminal voltage") of a single cell exhibited
before and after an endurance test.
[0026] FIG. 4B is a graph showing a relationship between a
differential pressure of reactant gases calculated from two
pressures at places upstream and downstream of a gas diffusion
layer of an electrode according to a fourth embodiment of the
present invention and an undervoltage (difference between terminal
voltages; ".DELTA. terminal voltage") of a single cell exhibited
with relative humidity of the reactant gases at 100% and 20%.
[0027] FIG. 4C is a diagram for explaining a method of measuring
the differential pressure for use in the fourth embodiment of the
present invention.
[0028] FIG. 5 is a graph showing a relationship between an adhesion
rate of a solid polymer membrane and electrode catalyst layer of a
membrane electrode assembly according to a fifth embodiment of the
present invention and an undervoltage (difference between terminal
voltages; "A terminal voltage") of a single cell exhibited before
and after an endurance test.
[0029] FIGS. 6A and 6B are schematic diagrams for explaining a
method of measuring a charge amount of an electrochemical surface
existing in an electrode catalyst layer by a cyclic
voltammetry.
[0030] FIG. 7A is a table showing measurements of terminal voltages
of a single cell using an electrode of Examples 1 through 3 and
Comparative examples 1 through 4.
[0031] FIG. 7B is a graph showing the measurements of FIG. 7B where
x-axis denotes a relative humidity of reactant gases, and y-axis
denotes a terminal voltage of a single cell with a current density
of 1 A/cm.sup.2.
[0032] FIG. 7C is a table showing measurements of differences
between terminal voltages (".DELTA. terminalvoltage") of a single
cell exhibited before and after an endurance test using an
electrode of Examples 1 through 3 and Comparative examples 1
through 6.
[0033] FIG. 8 is a schematic diagram of a single cell of a solid
polymer fuel cell including an electrode of a conventional
type.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] A description will be given of exemplary embodiments of the
present invention with reference to the drawings.
[0035] <General Structure: First Embodiment>
[0036] First of all, an exemplified general structure of a single
cell of a solid polymer fuel cell including an electrode (and
membrane electrode assembly) according to the present invention
will be described as a first embodiment of the present invention
with reference to FIG. 1.
[0037] The single cell 10 of the solid polymer fuel cell includes,
as shown in FIG. 1, a solid polymer membrane 1, and disposed at
both sides of the solid polymer membrane 1 are electrode catalyst
layers 2a, 2b, water-holding layers 3a, 3b, and gas diffusion
layers 4a, 4b in this order. These components 4a, 3a, 2a, 1, 2b.
3b, 4b make up a membrane electrode assembly (MEA). The single cell
10 further includes separator plates 5a, 5b which are disposed at
both sides of the MEA and serve to separate the MEA from MEAs of
other cells adjacent to the cell 10 and to provide channels for
supplying reactant gases (fuel gases and oxidant gases). In other
words, there is provided a pair of electrodes each include the gas
diffusion layer 4a (4b), the water-holding layer 3a (3b) and the
electrode catalyst layer 2a (2b), and the solid polymer membrane 1
is disposed between the electrodes. In the electrode, the electrode
catalyst layer 2a (2b) is disposed between the solid polymer
membrane 1 and the gas diffusion layer 4a (4b), and the
water-holding layer 3a (3b) is disposed between the gas diffusion
layer 4a (4b) and the electrode catalyst layer 2a (2b). The solid
polymer membrane 1 is sandwiched between the electrodes to form an
MBA, and the MEA is sandwiched by the separator plates 5a, 5b to
form a principal structure of the cell 10.
[0038] The solid polymer membrane 1 is an electrolyte membrane
having ion conductivity. In particular, perfluorosulfonic acid
polymer membrane may be employed, which are sold under brand names
such as Nafion.RTM., Flemion.RTM., Aciplex.RTM., etc. In the
present embodiment, Nafion.RTM. manufactured by E. I. du Pont de
Nemours and Company is employed. In order to have sufficient proton
conductivity exerted in the solid polymer membrane, the membrane
should retain a sufficient amount of water. However, protons are
hydrated in the membrane, and water is transferred from the anode
to the cathode by electroendosmosis. Thus, electrolysis at the
electrode, especially at the anode, is likely to dry. Accordingly,
reactant gases supplied through the separator plates 5a, 5b are
humidified in advance so as not to allow the solid polymer membrane
1 to dry.
[0039] The electrode catalyst layers 2a, 2b contain a catalyst, and
the catalyst for fuel gases and the catalyst for oxidant gases are
different in metal content. The catalyst most used for both fuel
gases and oxidant gases is platinum (Pt), but if carbon monoxide
could be included in the gases, poison-inhibitive metal is added to
the platinum because carbon monoxide would poison a platinum-based
catalyst. In the present embodiment, a platinum-based catalyst in
which platinum is supported by carbon black (carbon powders) is
used in the electrode for oxidant gases, and a catalyst in which
platinum and ruthenium are supported by carbon black is used in the
electrode for fuel gases. The electrode catalyst layers 2a, 2b may
further include an ion conductive polymer such as
polytetrafluoroethylene copolymers, perfluorosulfonic acid polymers
or the like. In the present invention, Nafion.RTM. is employed for
the ion conductive polymer. Moreover, a pore-making material PM is
further added to the mixture of the ion conductive polymer and the
catalyst-supporting carbon powder to make the layers porous and to
thereby facilitate drainage of water from the electrode catalyst
layers 2a, 2b. Any known materials usable as the pore-making
material may be used for giving a desired porous structure to the
electrode catalyst layers 2a, 2b, and a crystalline carbon fiber is
added as the pore-making material in the present embodiment. It is
however to be understood that the electrode catalyst layers 2a, 2b
according to the present invention are not limited to the above
composition.
[0040] The water-holding layers 3a, 3b are made by adding a
pore-making material PM to an ion conductive polymer, and possess
high water-retaining capability. In general, the ion conductive
polymer applicable thereto may be prepared by placing
polytetrafluoroethylene copolymer, polypyrrole, polyaniline, or the
like into a dispersion medium to form colloidal particles. Among
usable materials for the pore-making material PM are carbon, methyl
cellulose, carboxyl methyl cellulose, polyvinyl alcohol, cellulose,
polysaccharide, and other organic materials. In the present
embodiment, the pore-making material for the water-holding layers
3a, 3b is prepared by mixing a carbon black powder with a
crystalline carbon fiber. It is however to be understood that the
water-holding layers 3a, 3b according to the present invention are
not limited to the above composition as far as the composition can
exert a desirable water-retaining capability.
[0041] For the gas diffusion layers 4a, 4b, a porous,
current-collecting, and physical support layer, e.g., carbon paper
(having approximately 80% porosity) may preferably be used. In the
present embodiment, Teflon.RTM. (polyfluoroethylenepropylene) in
which carbon black powers are dispersed is applied to carbon paper,
to form the gas diffusion layers 4a, 4b.
[0042] The separator plates 5a, 5b include grooves serving as
channels for distributing reactant gases. The separator plates 5a,
5b may be made of carbon or metal of various kinds, one or more
materials may be selected as appropriate and used singly or in
combination to meet the particular purposes.
[0043] The electrode and membrane electrode assembly having the
above-described structure according to the first embodiment of the
present invention exert the following advantageous features:
[0044] (1) Addition of a pore-making material PM to the electrode
catalyst layers 2a, 2b to facilitate drainage of water from the
electrode catalyst layers 2a, and 2b makes it possible to prevent
flooding even under conditions in which reactant gases exhibit high
relative humidity. Thus, the fuel cell having the electrode
according to the present invention can maintain as high power
generation performance as could be achieved in the fuel cell unless
affected by flooding.
[0045] (2) Provision of the water-holding layer 3a (3b) for
enhancing a water-retaining capability of the gas diffusion layer
4a (4b) between the electrode catalyst layer 2a (2b) and the gas
diffusion layer 4a (4b) allows the gas diffusion layers 3a, 3b to
always hold water (moisture content) enough to maintain a
sufficient level of proton conductivity of the solid polymer
membrane 1. Thus, even under conditions where shortages of fuel
gases are encountered frequently, corrosion of carbon can be
prevented from proceeding in the catalyst electrode layers 2a, 2b
of the membrane electrode assembly MEA. Accordingly, the fuel cell
having the electrode according to the present invention can exert
improved power generation performance and durability.
[0046] Consequently, according to the present embodiment, an
electrode and membrane electrode assembly for a solid polymer fuel
cell having a stable power generation performance and high
durability such that the performance and the durability thereof are
not affected by change in relative humidity in reactant gases
supplied to the solid polymer fuel cell.
[0047] <Second Embodlment>
[0048] Next, a description will be given of a second embodiment of
the electrode and membrane electrode assembly according to the
present invention which includes a gas diffusion layer having
optimum moisture content with reference to FIG. 2. FIG. 2 is a
graph showing a relationship between a moisture content of a gas
diffusion layer of an electrode according to the second embodiment
of the present invention and an undervoltage (difference between
terminal voltages; "A terminal voltage") of a single cell exhibited
before and after an endurance test.
[0049] In FIG. 2, the abscissa denotes moisture content (mass %),
percentage of moisture content of the gas diffusion layer relative
to mass of the gas diffusion layer under dry conditions, and the
ordinate denotes a difference between terminal voltages (.DELTA.
terminal voltage) of a single cell obtained before and after the
endurance test. That is, the moisture content is given by Equation
(3): 2 MoistureContent = SM - DM DM .times. 100 [ % ] ( 3 )
[0050] where SM denotes mass of the gas diffusion layer under
saturation of water vapor pressure; DM denotes mass of the gas
diffusion layer under dry conditions.
[0051] [Endurance Test]
[0052] The endurance test was carried out in a mower as
follows:
[0053] First, electric current and voltage performance of a fuel
cell embodying the present invention as shown in FIG. 1 and a fuel
cell as shown in FIG. 8 prepared for comparison are measured.
Subsequently, an endurance test under transient hydrogen-deficient
conditions (load test) was conducted. In the endurance test under
transient hydrogen-deficient conditions, current applied to the
fuel cell were fluctuated between 0 A/cm.sup.2 and 1 A/cm.sup.2 at
regular intervals for 200 hours. The interval of fluctuation was 20
seconds from 0 A/cm.sup.2 to 1 A/cm.sup.2, and 1 A/cm.sup.2 to 0
A/cm.sup.2. The amount of gases supplied to the anode and the
cathode are configured so that the average utilization rates
thereof exhibit 200% and 50%, respectively. The difference between
terminal voltages exhibited before and after the aforementioned
endurance test is employed as A terminal voltage as shown in FIG.
2. It is understood that the increase in A terminal voltage
indicates the deterioration in performance of the membrane
electrode assembly. Hereupon, a practical range of the .DELTA.
terminal is determined as 30 mV or lower.
[0054] [Measuring Moisture Contents]
[0055] The moisture contents of the gas diffusion layer were
measured in a manner as follows:
[0056] (1) A gas diffusion layer having predetermined dimensions
and mass are put into a moisture content measuring instrument,
[0057] (2) The vapor pressure is varied, and the gas diffusion
layer is left until the mass becomes unchanged for each vapor
pressure.
[0058] (3) A mass of the gas diffusion layer having a stable mass
under a specific vapor pressure is measured using an electronic
balance, and a moisture content of the gas diffusion layer for each
vapor pressure is obtained by Equation (3).
[0059] (4) a sample of the gas diffusion layer having the
predetermined dimensions and mass is put into a thermo-humidistat
chamber, left for one hour, and taken out of the chamber. Water is
wiped off the sample and then the sample is weighed in the
electronic balance; a moisture content of the sample is obtained by
Equation (3).
[0060] As shown in FIG. 2, in a range of the moisture content up to
50%, the lower the moisture content, the higher the A terminal
voltage becomes gradually. This indicates that as the moisture
content in the gas diffusion layer decreases, drainage of water is
accelerated to such an extent that only the electrolysis of water
cannot produce sufficient hydrogen ion to be supplied to the
electrode catalyst layer, thus causing corrosion of carbon to
proceed in the electrode catalyst layer. On the other hand, in a
range of the moisture content beyond 90%, the higher the moisture
content, the higher the A terminal voltage becomes gradually. This
is because the increase of the moisture content over 90% causes
flooding to occur, which lowers the capability of supplying
reactant gases, thus expanding a region in which fuel gases are
insufficient.
[0061] In contrast, it is shown that in a range of the moisture
content between 50% and 90%, the A terminal voltage is stably
maintained at a sufficiently low level.
[0062] From the foregoing, the electrode or membrane electrode
assembly according to the second embodiment of the present
invention which basically has the same structure as described in
the first embodiment with reference to FIG. 1 includes a gas
diffusion layer having a preferable moisture content ranging
between 50% and 90% inclusive.
[0063] <Third Embodiment>
[0064] Next, a description will be given of a third embodiment of
the electrode and membrane electrode assembly according to the
present invention with reference to FIG. 3. The difference in
structure of the electrode according to the third embodiment from
the electrode according to the first and second embodiments is in a
water-repellent layer (not shown) for facilitating drainage of
water from the gas diffusion layer, which water-repellent layer is
provided between the water-holding layer and the gas diffusion
layer. The water-repellent layer is formed by mixing Teflon
dispersed solution with carbon black powders. To be more specific,
the electrode according to the third embodiment includes an
electrode catalyst layer, a water-holding layer, a water-repellent
layer, and a gas diffusion layer; among these components, the gas
diffusion layer is targeted for moisture-content adjustment so that
a stable power generation performance can be achieved.
[0065] A relationship between a moisture content of the gas
diffusion layer of the electrode and a terminal voltage of a single
cell is graphically shown in FIG. 3. Hereupon, the abscissa denotes
a moisture content (mass %), i.e., percentage relative to the dry
mass of the gas diffusion layer; and the ordinate denotes a
terminal voltage of the single cell. The moisture contents of the
gas diffusion layer were measured in the same manner and calculated
by Equation (3) as in the second embodiment.
[0066] As shown in FIG. 3, with the electrode according to the
present embodiment, the power generation performance of the cell
comes as follows:
[0067] (1) The terminal voltage of the cell represented when a
relative humidity of reactant gases is as high as 100% is higher
than that represented when the relative humidity is as low as
20%.
[0068] (2) If the electrode includes no water-repellent layer
and/or water-holding layer and the moisture content of the gas
diffusion layer is lower than 50%, then a desired level of the
terminal voltage (i.e., 0.6V)-cannot be derived from the cell.
[0069] (3) On the other hand, if the cell includes a water-holding
layer containing no pore-making material and the moisture content
of the gas diffusion layer is higher than 90%, then a desired level
of the terminal voltage can still be derived from the cell.
However, if the relative humidity of reactant gases is as high as
100%, then failure to add a pore-making material to the
water-holding layer leads to insufficient drainage of water, and
makes water stagnant in the cell, thereby lowering the terminal
voltage. Consequently, the power generation performance is affected
badly.
[0070] (4) If the moisture content of the gas diffusion layer
ranges between 50% and 90%, i.e., the electrode is provided with
both of the water-holding layer and the water-repellent layer, then
stable power generation performance unsusceptible to change in
humidity can be achieved irrespective of whether or not the
relative humidity varies in reactant gases.
[0071] <Fourth Embodiment>
[0072] Next, a description will be given of a fourth embodiment of
the electrode and membrane electrode assembly according to the
present invention with reference to FIGS. 4A through 4C. Unlike the
electrodes according to the second and third embodiments of the
present invention in which the moisture content of the gas
diffusion layer is adjusted appropriately to stably achieve an
adequate level of power generation performance, the fourth
embodiment of the electrode is configured to adjust a differential
pressure of reactant gases calculated from two pressures measured
at places upstream and downstream of the gas diffusion layer to a
specific range, i.e., between 60 mmH.sub.2O and 120 mmH.sub.2O
inclusive, so that an adequate level of power generation
performance can be stably achieved.
[0073] FIG. 4A is a graph showing a relationship between a
differential pressure of reactant gases calculated from two
pressures at places upstream and downstream of the gas diffusion
layer and an undervoltage (difference between terminal voltages;
".DELTA. terminal voltage") of a single cell exhibited before and
after an endurance test. FIG. 4B is a graph showing a relationship
between a differential pressure of reactant gases calculated from
two pressures at places upstream and downstream of the gas
diffusion layer and an undervoltage (difference between terminal
voltages; ".DELTA. terminal voltage") of a single cell exhibited
with relative humidity of the reactant gases at 100% and 20%. FIG.
4C is a diagram for explaining a method of measuring the
differential pressure. As shown in FIG. 4C, the differential
pressure AP of the gas diffusion layer can be determined by
comparing two pressures measured at places upstream and downstream
of the gas diffusion layer when a predetermined flow rate (e.g. 500
L/cm.sup.2/min.) of the reactant gases pass through the gas
diffusion layer.
[0074] As seen from FIG. 4A, in a range of the differential
pressure AP below 60 mmH.sub.2O, the lower the differential
pressure .DELTA.P, the higher the .DELTA. terminal voltage becomes.
In this range, the gas diffusion layer has water-draining
capability enhanced too much to supply hydrogen ion only through
the electrolysis of water, and thus corrosion of carbon proceeds.
On the other hand, in a range of the differential pressure .DELTA.P
beyond 120 mmH.sub.2O, the higher the differential pressure
.DELTA.P, the lower the gas supplying capability becomes due to
flooding, thus expanding a region in which fuel gases are
insufficient. In contrast, in a range of the differential pressure
.DELTA.P between 60 mmH.sub.2O and 120 mmH.sub.2O inclusive, the
.DELTA. terminal voltage is stably maintained at a low level.
[0075] It is understood that a larger value of the .DELTA. terminal
voltage indicates a larger difference between terminal voltages
before and after the endurance test as described above; therefore,
the magnitude of the .DELTA. terminal voltage indicates the degree
of undervoltage of the terminal voltage, i.e., decrease in output
of the solid polymer fuel cell.
[0076] Turning to FIG. 4B, the influence of change in relative
humidity will be described below. In FIG. 4B, the abscissa denotes
a differential pressure of reactant gases calculated from two
pressures at places upstream and downstream of the gas diffusion
layer, and the ordinate denotes a difference between terminal
voltages (".DELTA. terminal voltage") of a single cell exhibited
with relative humidity of the reactant gases at 100% and 20%.
[0077] As shown in FIG. 4B, with the electrode according to the
present embodiment, the power generation performance of the cell
comes as follows:
[0078] (1) In a range of the differential pressure .DELTA.P of the
gas diffusion layer lower than 6 mmH.sub.2O, if the amount of
pre-making material added to the water-holding layer is much, the
water-retaining capacity becomes little. Therefore, 20% relative
humidity of reactant gases cannot serve to maintain the ion
conductivity of the solid polymer membrane, and thus the difference
between terminal voltages (".DELTA. terminal voltage") exhibited
with relative humidity of the reactant gases at 100% and 20%
becomes greater.
[0079] (2) On the other hand, in a range of the differential
pressure AP of the gas diffusion layer higher than 120 mmH.sub.2O,
failure to add a pore-making material to the water-holding layer
leads to insufficient drainage of water, and makes water stagnant
in the cell, thereby causing flooding in the cell. Consequently,
the difference between terminal voltages (".DELTA. terminal
voltage") associated with change in relative humidity of the
reactant gases becomes greater.
[0080] (3) If the differential pressure .DELTA.P of the gas
diffusion layer ranges between 60 mmH.sub.2O and 120 mmH.sub.2O
inclusive, the difference between terminal voltages (".DELTA.
terminal voltage") exhibited with relative humidity of the reactant
gases at 100% and 20% can be maintained within a preferable range,
i.e., 35 mV or lower. Accordingly, stable power generation
performance unsusceptible to change in humidity can be achieved
irrespective of whether or not the relative humidity varies in
reactant gases.
[0081] From the foregoing, the electrode or membrane electrode
assembly according to the fourth embodiment of the present
invention which basically has the same structure as described in
the first embodiment with reference to FIG. 1 includes a gas
diffusion layer exhibiting a preferable differential pressure of
the reactant gases calculated from two pressures measured at places
upstream and downstream of the gas diffusion layer when a
predetermined flow rate of the reactant gases pass through the gas
diffusion layer ranging between 60 mmH.sub.2O and 120 mmH.sub.2O
inclusive.
[0082] <Fifth Embodiment>
[0083] Next, a description will be given of a fifth embodiment of
the membrane electrode assembly according to the present invention
with reference to FIGS. 5, 6A and 6B. FIG. 5 is a graph showing a
relationship between an adhesion rate of a solid polymer membrane
and electrode catalyst layer and an undervoltage (difference
between terminal voltages; ".DELTA. terminal voltage") exhibited
before and after the endurance test as in the second embodiment.
FIGS. 6A and 6B are schematic diagrams for explaining a method of
measuring a charge amount of an electrochemical surface existing in
the electrode catalyst layer by a cyclic voltammetry.
[0084] The membrane electrode assembly provided as the fifth
embodiment of the present invention basically has the same
structure as described in the first embodiment with reference to
FIG. 1, and has a preferable adhesion rate of the solid polymer
membrane 1 and electrode catalyst layers 2a, 2b, i.e., a percentage
of a charge amount of catalytic substances existing on an interface
between the solid polymer membrane 1 and the electrode catalyst
layer 2a (2b) to a charge amount of all catalytic substances
existing in the electrode catalyst layer 1 is configured to be 15%
or greater. Hereupon, the above charge amounts used to determine
the adhesion rate are measured by the cyclic voltammetry, which
will be described in detail below with reference to FIGS. 6A and
6B.
[0085] As shown in FIG. 6A, first, by means of a typical process of
the cyclic voltammetry, a charge amount of all the catalytic
substances existing in the electrode catalyst layers of the
electrodes is measured. More specifically, humidifying gases are
supplied to the anode A and the cathode C until water is
distributed throughout the whole cell; then, a charge amount of
electrochemical surfaces of all catalyst particles is measured.
[0086] Next, as shown in FIG. 6B, humidifying gases are supplied to
the anode A only, and a charge amount of electrochemical surfaces
of the catalyst particles is measured. When only the anode A is
humidified, water transferred from the anode A is distributed only
to conducting channels at a cathode A side of the solid polymer
membrane 1. Accordingly, electrochemical surfaces of catalysts
existing on an interface between the solid polymer membrane and the
electrode catalyst layer, i.e., a charge amount thereof can be
evaluated.
[0087] The adhesion rate used in the present embodiment can be
given by Equation (4) below: 3 AdhesionRate = SC TC .times. 100 [ %
] ( 4 )
[0088] where SC denotes a charge amount of catalytic substances
existing on an interface between the solid polymer membrane and the
electrode catalyst layer, and TC denotes a charge amount of all
catalytic substances existing in the electrode catalyst layer.
[0089] The more the amount of catalysts existing on the interface
between the solid polymer membrane and the electrode catalyst layer
(L e., the higher the adhesion rate), the more efficiently the
catalysts can be utilized.
[0090] As shown in FIG. 5, in a range of the adhesion rate below
15%, as the adhesion rate increases, the .DELTA. terminal voltage
sharply drops, and then (in a range of the adhesion rate of 15% or
higher) the A terminal voltage gradually decreases. To be more
specific, in a range of the adhesion rate below 15%, a reverse
diffusion amount of water generated at the cathode side decreases,
and thus hydrogen ion cannot sufficiently be supplied only through
the electrolysis of water. Therefore, corrosion of carbon proceeds
in the electrode catalyst layer. On the other hand, in a range of
the adhesion rate of 15% or higher, this phenomenon does not take
place; thereby sufficient durability can be maintained.
OPERATIVE EXAMPLES
[0091] Next, to verify the facts acquired from the above-discussed
first through fifth embodiments, a description will now be given of
specific examples of the electrodes and membrane electrode
assemblies with reference to FIGS. 7A-7C.
[0092] First, a method of fabricating components or layers
constituting a single sell of the solid polymer fuel cell prepared
in the following examples will be described
[0093] (1) Electrode Catalyst Layer
[0094] 1-a) Fabrication of Cathode (Oxidant Gas Terminal)
[0095] Crystalline carbon fiber (VGCF; manufactured by Showa Denko
Kabushiki Kaisha) is mixed with ion conductive polymer
(Nafion.RTM., SE20192; manufactured by E. I. du Pont de Nemos and
Company) 35 g and platinum supporting carbon powders (TEC10E50E;
manufactured by Tanaka Kikinzoku Kogyo Kabushild Kaisha) 2.5 g with
mass ratio of carbon black to platinum being 50:50 to form a
catalyst paste for the cathode. The catalyst paste is applied to a
FEP (fluoroethylene propylene tetrafluoroethylene-hexafluoro
propylene copolymer) sheet so that the amount of platinum on the
FEP sheet is 0.3 mg/cm.sup.2. The FEP to which the catalyst paste
is applied is then dried to form an electrode catalyst layer sheet
CA.
[0096] 1-b) Fabrication of Anode (Fuel Gas Terminal)
[0097] Ion conductive polymer Nafion.RTM., SE20192; manufactured by
E. I. du Pont de Nemours and Company) 36.8 g and platinum/ruthenium
supporting carbon powders (TEC61E54; manufactured by Tanaka
Kiknzoku Kogyo Kabusfild Kaisha) 10 g with mass ratio of carbon
black to Pt--Ru catalyst being 1:1 are mixed together to form a
catalyst paste for the anode. The catalyst paste is applied to a
FEP (fluoroethylene propylene
tetrafluoroethylene-hexafluoropropylene copolymer) sheet so that
the amount of platinum on the PEP sheet is 0.15 mg/cm.sup.2. The
PEP to which the catalyst paste is applied is then dried to form an
electrode catalyst layer sheet AN.
[0098] The electrode catalyst layer sheet CA and the electrode
catalyst layer sheet AN are transferred to and bot pressed with a
solid polymer membrane (electrolyte membrane) to form a membrane
electrode assembly (MEA in a broad sense of the term) according to
the present invention.
EXAMPLE 1
[0099] A water-holding layer is formed using a paste for a
water-holding layer prepared by mixing crystalline carbon fiber
(VCEF; manufactured by Showa Denko Kabushiki Kaisha) 2.5 g with ion
conductive polymer (Nafion.RTM. SE20192; manufactured by E. I. du
Pont de Nemours and Company) 25 g, and carbon black powders (Ketjen
Black; manufactured by Cabot Corporation) 5 g.
[0100] A water-repellent layer is formed using a paste for a
water-repellent layer prepared by mixing carbon black powders
Vulcan XC75, manufactured by Cabot Corporation) 18 g with
Teflon.RTM. (polyfluoroethylenepropylene) dispersed solution
(L170J; manufactured by Asahi Glass Co. Ltd.) 12 g.
[0101] Next, 2.3 mg/cm.sup.2 of the above paste for a
water-repellent layer is applied to carbon paper (TGPO60;
manufactured by Toray Industries, Inc.) rendered water repellent in
advance to form a gas diffusion layer (with water-repellent layer),
and 0.3 mg/cm.sup.2 of the paste for a water-holding layer is
applied to the gas diffusion layer.
[0102] Lastly, between two gas diffusion layers coated with the
water-holding layer are disposed the above MBA to form a membrane
electrode assembly (MEA) for a single cell as defined in the
present invention.
EXAMPLE 2
[0103] A single cell was fabricated by the same process as in
Example 1 except that the amount of the paste for water-holding
layer applied to water-repellent carbon paper (TGP60; manufactured
by Toray Industries, Inc.) was 0.4 mg/cm.sup.2, which was larger
than that applied in Example 1.
EXAMPLE 3
[0104] A single cell was fabricated by the same process as in
Example 1 except that the amount of the paste for water-holding
layer applied to water-repellent carbon paper (TGP60; manufactured
by Toray Industries. Inc.) was 0.2 mg/cm.sup.2, which was smaller
than that applied in Example 1.
COMPARATIVE EXAMPLE 1
[0105] A single cell was fabricated by the same process as in
Example 1 except that the amount of the crystalline carbon fiber
added to the water-holding layer was 3.5 g, which was larger than
that applied in Example 1.
COMPARATIVE EXAMPLE 2
[0106] A single cell was fabricated by the same process as in
Example 1 except that the amount of the crystalline carbon fiber
added to the water-holding layer was 0 g, i.e. no crystalline
carbon fiber was added.
COMPARATIVE EXAMPLE 3
[0107] A single cell was fabricated by the same process as in
Example 1 except that the water-holding layer as in Example 1 was
not applied, but only the paste for water-repellent layer was
applied.
COMPARATIVE EXAMPLE 4
[0108] A single cell was fabricated by the same process as in
Example 1 except that neither the paste for water-holding layer nor
the paste for water-repellent layer as in Example 1 was applied,
but only carbon paper (TGPO60; manufactured by Toray Industries,
Inc.) rendered water repellent in advance as in Example 1 was used
as the gas diffusion layer for forming a membrane electrode
assembly.
COMPARATIVE EXAMPLE 5
[0109] A single cell was fabricated by the same process as in
Example 1 except that pressure for hot pressing the electrode sheet
to the solid polymer membrane was
COMPARATIVE EXAMPLE 6
[0110] A single cell was fabricated by the same process as in
Example 1 except that pressure for hot pressing the electrode sheet
to the solid polymer membrane was 30 kg/cm.sup.2.
[0111] Results of measurement of power generation performance with
relative humidity varied in reactant gases supplied to single cells
of Examples 1-3 and Comparative examples 1-4 are shown in FIGS. 7A
and 7B. The conditions of operation were as follows: (1) fuel gases
and oxidant gases were humidified at the same relative humidity;
(2) operating temperature was maintained at 75.degree. C.; (3)
pressure of gases supplied to the electrodes (fuel-gas terminal and
oxidant-gas terminal) were both 200 kPa; and (4) terminal voltages
were measured with current density of the electrodes of the cell at
1 A/cm.sup.2. The results can be evaluated as follows:
[0112] (1) With the cells in Examples 1-3, desirable terminal
voltages which were higher than 0.6V were observed. The moisture
contents of the gas diffusion layers were within the range of
48.6-90.4 mass %; it is thus evaluated that the power generation
performance was stably maintained at a sufficient level
irrespective of humidity of the supplied reactant gases. It turned
out that provision of the water-holding layer and the
water-repellent layer serve to produce a desirable level of power
generation performance (i.e., 0.6 V or higher) irrespective of
humidity of the reactant gases.
[0113] (2) Comparative Example 1 used a single cell including a
water-holding layer containing 0.4 mg/cm.sup.2 of crystalline
carbon fibers the amount of which was larger than Example 1, and a
water-repellent layer. As shown in FIG. 7B, if the relative
humidity of the reactant gases is 40% or higher, the terminal
voltage can fall within A desirable range of 0.6V or higher.
[0114] (3) Comparative Example 2 used a single cell including a
water-holding layer containing 0 g of crystalline carbon fibers the
amount of which was smaller than Example 1, and a water-repellent
layer. As shown in FIG. 7B, if the relative humidity of the
reactant gases is lower than 40%, the power generation performance
exhibits an excellent level, but if the relative humidity of the
reactant gases is 60% or higher, the power generation performance
becomes insufficient, because water cannot properly be drained out
of the water-holding layer, contrary to Comparative Example 1.
[0115] (4) Comparative Example 3 used a single cell including no
water-holding layer bat a water-repellent layer. As shown in FIG.
7B, unless the relative humidity is high, a desirable level of the
terminal voltage, i.e., 0.6V or higher, cannot be obtained.
[0116] (5) Comparative Example 4 used a single cell including
neither water-holding layer nor water-repellent layer but with only
carbon paper rendered water repellent in advance used as a gas
diffusion layer. As shown in FIG. 7B. Comparative Example 4 is most
susceptible to the change in relative humidity among the
comparative examples. The terminal voltage cannot reach a desirable
level, i.e., 0.6V or higher, unless the relative humidity is as
high as 100%, as in Comparative Example 3.
[0117] Next, results of measurement of differences of terminal
voltages (".DELTA. terminal voltage") before and after the 200-hour
endurance test carried out in a manner as described above using
single cells of Examples 1-3 and Comparative Examples 1-6 will be
explained with reference to FIG. 7C. The conditions of operation
were substantially the same as above results of measurement
described with reference to FIGS. 7A and 7B. The results can be
evaluated as follows:
[0118] (6) The cells of Examples 1-3 exhibited a sufficiently low
level of .DELTA. terminal voltages, and the power generation
performance was not affected by the 200-hour endurance test.
[0119] (7) The cells of Comparative Examples 14 prepared with the
differential pressures outside the desirable range defined by the
present invention exhibited a higher level of .DELTA. terminal
voltages beyond a desirable range defined by the present invention.
Therefore, it turned out that the performances of the cells were
affected badly by the improper differential pressure.
[0120] (8) The cells of Comparative Examples 5 and 6 prepared with
the adhesion rates outside the desirable range defined by the
present invention exhibited a higher level of .DELTA. terminal
voltages beyond a desirable range defined by the present invention.
Therefore, it turned out that the performances of the cells were
affected badly by the endurance test.
[0121] Although the preferred embodiments of the present invention
have been described above, the present invention is not limited to
the above exemplified embodiments, and various modifications and
changes may be made in the present invention without departing from
the spirit and scope thereof.
* * * * *